In the production of precision metal objects, expendable patterns are many times utilized to generate mold forms usable in metal casting processes. The molds are formed about an expendable pattern, whereby the pattern can be disposed of after formation of the mold to generate a cavity into which a molten metal may be poured, thereby producing the metal castings. The metal castings are often intricate in their geometry and require great precision in characteristics such as dimension and surface finish so as to be acceptable for their intended use. The processes generally used for producing precision metal objects from expendable patterns are investment casting techniques.
In the investment casting process, a ceramic shell is formed about an expendable pattern by coating of the pattern with a ceramic dip coat composition. After coating, the wetted surface of the pattern may be covered by a stucco coat of ceramic particles and air dried, after which a number of similar dip coats and stucco coats may be similarly applied until a shell of sufficient thickness and strength has been formed about the expendable pattern. Once the mold form is fabricated in this manner, the expendable pattern is removed, leaving the mold form and cavity which may then be used in a metal casting process.
Another process has been termed the mono-shell process, which includes the steps of building up a shell by alternate dip and stucco coats, wherein the formed shell is then given a final coat of a dip coat composition containing an eutectic material having a lower maturing temperature than the ceramic materials in the stucco or dip coats. Upon subsequent heating of the composite shell structure to a high temperature for removal of the expendable pattern, the eutectic material is made to flow inwardly into the mold form for better sealing thereof which helps to eliminate fracturing or other affects caused by the elevated temperatures used to remove the expendable pattern. The mold form generated in this process will have a greater strength to allow handling in the casting process without additional steps being necessary to strengthen the mold form.
In the investment casting process, an expendable pattern is also utilized, and may be coated with built-up layers of dip coat and stucco in a manner similar to that previously described. The mold form generated in this manner may then be invested within a matrix of ceramic material to provide additional support for the mold form to allow handling without deterioration thereof. Alternatively, an expendable pattern may be covered with a suitable investment material such as a ceramic, which is then permitted to solidify to generate the desired mold form having intricate geometry and great precision as required. In the investment process, the expendable pattern or a composite pattern and the mold shell formed therearound may be mounted within an enclosure and exposed to a composition of a ceramic material which will set up and form a rigid exterior about the expendable pattern or composite mold form. The ceramic material may then be heated for a predetermined amount of time in a heating cycle to fire the ceramic investment material of the mold form. For use of the mold form for casting of metal objects, the expendable pattern material may then be removed. Conventionally, during the heating cycle, the expendable pattern is removed either by being reduced to a molten state so as to flow from the mold form or by burning the expendable pattern material when heated to the temperature of combustion, or both. The mold form will then comprise the investment as a matrix, and an inner lining of the built-up layers formed of the dip coat and stucco materials, which is then available for casting of the precision metal pieces.
In the above mentioned processes used for producing precision metal objects by metal casting techniques, the expendable or sacrificial mold patterns are conventionally comprised of low melting point waxes, which when heated to a sufficient degree will be reduced to a flowable state for removal from the formed mold. In many situations, long term, low temperature melt-out cycles are utilized in an effort to minimize fracturing of the mold which may result from more radical temperature variations or the exposure of the mold form to higher temperatures. It should be recognized that if the material used in the expendable pattern is heated to the degree of combustion, expansion of the materials upon combustion may also result in mold spoilage by deformation of the precise geometrical characteristics and dimensions required in the mold or fracturing thereof. It has been found that the use of low melting point waxes utilized in the fabrication of large mold patterns are unsuitable in many respects as the material may deform either during the fabrication of the patterns or handling thereof subsequent to fabrication, and be extremely expensive to hand fabricate. The special problems frequently encountered when attempting to produce large precision parts by the lost wax casting processes generally stem from the inadequacies of the materials and/or processes used to form the expendable or sacrificial mold patterns. Attempts to overcome the deficiencies of using fabricated low melting point waxes as the pattern materials have included the use of more structurally competent plastic materials has been attempted. Such plastic materials are advantageous in that the structural integrity is maintained during fabrication and handling as the materials are not readily deformable, and such plastic materials may be formed into a variety of complex geometries with great precision as required. The use of plastic materials in the preparation of expendable patterns also allow the fabricated pattern to be composed of a plurality of plastic parts which may be coupled to one another to generate the final pattern. For many metal castings which are intricate in their geometry and require great precision as to dimension and surface finish, the use of such plastic materials may also simplify the fabrication of the mold pattern. For complex expendable mold patterns, such as, for instance, a turbine wheel be constructed of a soluble or low melting point wax injected into a suitable mold configured for the individual segments or portions thereof. These individual wax segments may be fused together in more elaborate assembly fixtures, requiring labor intensive activities instances, a wax material handled in this manner will become deformed, and the elaborate assembly components for complex castings may be deficient in their capacity to form a complex and precision casting with acceptable tolerance specifications. The use of plastic patterns overcome some of the disadvantages with respect to the fabricated low melting point waxes and allow the construction of extremely complex patterns in a straightforward manner without the possibility of pattern spoilage by means of deformation or fracturing during handling thereof.
Although expendable patterns utilizing plastic materials overcome some of the disadvantages of the fabricated low melting point waxes, a major deficiency in the use of such plastics is found in the potential inability to remove the plastic pattern from the mold form easily and effectively without incurring mold spoilage. It has been found that the application of heat in an attempt to remove such plastic patterns may result in significant expansion of the plastic materials within the mold cavity resulting from the change from solid to vapor in a very short period of time, rendering the use thereof unsuitable for most lost wax casting processes. The problem is manifested in the propensity of the plastic material, when subjected to heat, to change from the solid state to the gaseous state in a very short period of time, which results in expansion of the materials within the mold cavity, and which in turn will normally fracture or otherwise damage the mold form. Although the application of heat during a heating cycle can reduce such plastic materials to a molten state so as to flow from the mold cavity, the range of temperatures at which the plastic material becomes molten or liquified is extremely small, and the heating cycle must be controlled with great accuracy to avoid the thermal reaction which makes the use of such plastic materials difficult and operationally complex.
Another attempt to avoid the inherent disadvantages of the use of low melting point waxes in the fabrication of expendable patterns for metal casting techniques, especially with respect to large or particularly intricate patterns, is shown in U.S. Pat. No. 3,063,113. In this prior patent, the problems described above with respect to pattern materials having the potential for expansion within the mold cavity or deformation due to inadequate structural integrity, were stated to be overcome by fabricating the expendable pattern from a combination of a base pattern preferably formed of a plastic material, having a thin coating on its surface of another material which had a melting point below the temperature at which significant deformational changes due to expansion would take place in the base material. In such a construction, the thickness of the coating material such as a lower melting point wax, was chosen based upon the amount of expansion expected in the base plastic material. During the heating cycle, the low melting point material will expand minimally before becoming molten thus not creating any forces which are detrimental to the mold, and will flow from the mold cavity to leave sufficient space between the mold form and the base material of the pattern to allow expansion of the base material without exerting any pressure on the casting mold. Although such a process of fabricating the expendable mold did in part solve some of the deficiencies with respect to prior techniques, significant problems still exist with respect to the use of materials which have the propensity for violent expansion within the mold cavity, and can still result in mold spoilage. Additionally, the technique as described in this prior patent is difficult in operation to obtain the proper geometry and precision with respect to the mold form dimensional and surface finish characteristics. The inaccuracy with which the final expendable pattern is formed may result in unacceptable mold forms, thereby decreasing the yield of acceptable castings.